The exhaust lines of engine motor vehicles of recent design are equipped with an exhaust after treatment device such as a particulate filter or a NOx trap to reduce their emissions of solid pollutants such as nitrogen oxides (NOx) and sulfur oxides (SOx), the exhaust after treatment device being intended to store such pollutants.
An exhaust after treatment device such as e.g. a diesel particulate filter (DPF) does not have an unlimited storage capacity, and the collected soot must be eliminated periodically and/or regularly to prevent e.g. the particulate filter (PF) from becoming clogged and to return it to its nominal efficiency. Moreover, clogging of the PF gradually creates a back-pressure that degrades the operation of the engine. This elimination of soot, referred to as “particulate filter regeneration”, may be effected by heating the filter to a temperature higher than the combustion temperature of soot (which is normally around 550° C.) by means of the exhaust gases flowing therein.
In a standard manner, the engine-out exhaust gas temperature can be increased for the purpose of PF regeneration by using a regeneration specific fuel injection pattern. In such injection pattern, the execution timing of the main fuel injection is retarded later than compression stroke top dead centre (TDC), and is typically preceded by a pilot injection about TDC.
Additionally, an oxidation catalyst may be installed upstream of the PF, the oxidation catalyst being adapted to generate an exotherm upstream of the PF, which increases the temperature of the exhaust gases and thus assists in the regeneration. A post injection may thus be performed to increase the quantity of available hydrocarbons in the exhaust gases, the hydrocarbons being in turn converted by the oxidation catalyst through the exothermic reaction, heating the exhaust gases to a temperature above 550° C.
As it is well known, in the conventional engine control, the engine is mainly operated in a lean combustion mode, which is intended to achieve a desired engine power. In today's engines, in normal engine operation the combustion mode performs fuel injection in accordance with a pattern comprising one or two pilot injections followed by a main injection near a compression top dead centre (TDC). The energy, which is generated by combustion of the injected fuel, is converted into the engine power at high efficiency.
Then, when a need for regeneration of the DPF is determined, the combustion mode is switched to a regeneration specific combustion mode with retarded multiple injections to provide an increase in the temperature of the exhaust gases and assist in regeneration process of the DPF. A typical regeneration pattern comprises a significantly retarded main injection, preceded by one or two pilot injections after compression TDC. As compared to the normal combustion mode, the retarded, regeneration specific multi-injection pattern should achieve the same engine torque for a given engine speed and load condition with a concurrent increase in the amount of waste heat, which thus requires additional fuel.
Then, to further increase the amount of waste heat, the injection pattern in regeneration combustion mode may comprise a post-injection, after the main injection, to increase the amount of uncombusted HC in the exhaust gas and thereby generate an exotherm in the oxidation catalyst.
Accordingly, the control of engine combustion during a DPF regeneration event consists in finding adequate injection parameters, namely: number of injection events per cycle, injection timing, fuel quantities, to provide the desired torque and waste heat while taking into account emission levels and ensuring engine stability and driveability. In this last respect, the switch from normal combustion to regeneration-specific combustion mode should not sensibly alter the driveability of the vehicle.